Rays are nice looking and pretty cool, but I would probably never get one. Main reason being you can get badly hurt from getting stung. It seems more difficult to avoid than getting bit by say, a piranha. Or maybe it's just me.
I must say that almost every fish that can be kept in a tank can be made interessting with right conditions. I dont want to say that "I would never keep this or that" because I have too many ideas on badass tanks that Im gonna setup some day. My latest project for example includes a large school of cory cats wich if you have asked a year ago never thought on having. My personal goal with fishkeeping are to create the ultimate tank that would make me say: Ok thats it! no more tanks for me! No one can do a better tank than this!
1) Asian Arowana and 2) pacu - all species.
1) AA are a great fish in their natural state. But breeders have ruined the good genetic lines by trying to market odd color phases and things of that sort for profit.
2) Pacu are big and ugly and nobody seems to get that you can't keep them in small fish tanks like Walmart, Petco and Petsmart tell you. Even most of the die hard pacu owners are keeping stunted fish. I get sick of being asked to take them in when people can't house a 60 lb fish anymore.